Person-borne concealed weapon or contraband detection at long distance is an urgent national security need, but presently there is no adequate solution available. For example, plastic objects, such as handguns, may not be detected by conventional metal detectors. Terahertz radiation is of great interest for this application because of its ability to penetrate clothing and provide resolution on the order of millimeters. Although this does not have the resolution of X-rays, the energy is low enough to not be ionizing, avoiding any health concerns. Two primary approaches are used from imaging at these frequencies, active and passive. Passive imagers capture differences in black body temperatures between a person an any concealed items. See, for example, Thruvison Systems Limited, “ThruVision standoff people screening—Digital Barriers” February 2012. These systems are limited in sensitivity due to the minor temperature and emissivity differences that define the image and improvements in image quality are primarily made by cryogenically cooling the system. These systems can also be large (e.g., having volumes approaching a cubic meter) and heavy (e.g., having masses of tens of kilograms).
In contrast, active imagers paint the target with a low-power terahertz signal and form the image from the energy reflected from the target. For most security screening applications, near video rates (>4 Hz) are required to allow for adequate security checkpoint throughput. To scan, the active radar mechanically steers a single beam across the target area in a serpentine pattern. Currently this mechanical scanning limits the frame rate of the system to approximately 1 fps for a 0.4 m×0.4 m target area. This is set by the mechanical behavior of the scanning mirror, which begins to deteriorate the angular accuracy of the optical system when the mirror is scanned faster. To overcome this limitation, an array of transceivers can be used to reduce the required angular rotation in one of the scanning dimensions. If an N-transceiver array is used, the frame rate can be increased by a factor of N since the angle required to scan the mirror is reduced.
There is a need for improved multi-pixel terahertz imaging systems that are inexpensive and convenient to fabricate.